I am reading the following line from a book:

The reasons that modems are so slow is that telephones were invented for carrying the human voice and the entire system has been carefully optimized for this purpose. Data have always been stepchildren. At the point where each analog signal ( for example analog telephones ) from the end-user ends in the end office, the wire runs through a filter that attenuates all frequencies below 300 Hz and above 3400 Hz. This bandwidth although 3100 Hz is quoted as 4000 Hz. - Computer Networks, Tannenbaum.

My question is if the frequencies below 300 Hz and above 3400 Hz is attenuated, then how can a voice signal transmit on the wire, because the audio signal has a bandwidth of 20 Hz to 20 Khz.

Maybe I do not understand, what is being filtered at the end office.

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 17, 2017 at 4:30

1 Answer 1


the audio signal has a bandwidth of 20 Hz to 20 Khz.

Humans have a hearing range of somewhere around that, though it varies between individuals.

But you don't need to pass anything like that range for intelligible speech. The important components of speech are contained in a relatively narrow band and limiting the bandwidth makes it much easier to control noise, interference and reflections in the network. Within the network a lower bandwidth means you can cram more phone calls down the same trunk.

  • So, all the speech signals can be crammed upto 3400 Hz. So, if I take an oscillocope on the sending end and the hearing end, then all the higher frequencies above 3400 Hz would vanish on the oscilloscope at the hearing end, even if they were present at the sending end? Aug 17, 2017 at 9:46

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